A pulse emitted from the sound or ultrasound sensor is reflected on the surface of the fill substance. The travel time of the pulse from the sensor to the surface and back is determined and from that the fill level, or fill height, is determined.
Such sound or ultrasound sensors are applied in many branches of industry, e.g. in the food industry, the water and wastewater sectors, and in chemicals.
In almost all areas of application, it is required that the sensors exhibit a radiation characteristic having a small beam angle for the main sound lobe and, at the same time, have small side lobes.
The beam angle of the sensor is essentially determined by the diameter of the front surface and the frequency. The sine of the beam angle of the radiated sound lobe equals the quotient of the wavelength of the radiated sound or ultrasound wave and the diameter of the front surface of the radiating element. Thus, to obtain a sound lobe of small beam angle, a large diameter needs to be used.
On the other hand, one achieves a good radiation characteristic with small side lobes by a bending shape of a radiating element, whose amplitude distribution corresponds approximately to a Gauss function and for which, additionally, the phase of the oscillation is the same over the entire surface. The larger the half-value width of this Gauss curve, the narrower the main lobe. It thus makes sense to produce an oscillation deflection shape, in which the available radiating surface is optimally utilized.
DE-C 42 33 365 discloses a sound or ultrasound sensor for transmission and/or reception of sound or ultrasound, having                a piezoelectric element for producing and/or receiving sound or ultrasound through the floor,        a matching layer between the piezoelectric element and the floor, and        a metal ring gripping around the piezoelectric element with a force-transmitting, and interlocking, fit.        
The ring and piezoelectric element thus form a unitary, oscillating, oscillation structure. In such case, therefore, the larger, outer diameter of the ring is used for calculating the beam angle of the sound lobe, and not the diameter of the piezoelectric element.
Additionally, it makes sense, also, to isolate sound or ultrasound oscillations from adjoining housing portions. On the one hand, in the case of a sympathetic oscillation of the housing wall, sound or ultrasound pulses can be transmitted from, and received by, the wall itself. This can lead to interference echoes. On the other hand, the sound or ultrasound can be transmitted as structure-borne sound to the housing and, from there, to a holder of the sensor and possibly even to further structural components at the location of use. This can likewise lead to significant interference signals.